1. Field of the Invention
This invention relates to the wire bonding technology in general. More specifically, the present invention relates to a method and apparatus for electrically connecting between two different portions of a workpiece (e.g. a leadframe or printed circuit board to which electronic components are mounted) by a thin metal wire.
2. Description of the Prior Art
In electronic circuits or components, it is usually necessary to electrically connect between two different portions of a workpiece. For instance, a lead of a semiconductor chip device need be electrically connected to a relevant bonding pad of the chip body through a metal wire. Further, a semiconductor chip mounted on a circuit board need be electrically connected through a metal wire to a relevant portion of a conductor pattern printed on the circuit board. An operation of bonding the metal wire for these purposes is called "wire bonding".
A typical prior art wire bonding method is disclosed in Japanese Patent Application Laid-open No. 62-136836 for example. The wire bonding method disclosed in this Japanese document comprises the following steps.
(1) A capillary tool, through which a metal wire is supplied longitudinally, is brought to a position directly above a first bonding portion (e.g. a semiconductor chip) of a workpiece (e.g. an electronic component) with a spark torch arranged between the lower end of the wire and the first bonding portion.
(2) Then, the lower end of the wire is melted into a ball by causing a spark discharge between the torch and the wire lower end.
(3) Then, the spark torch is retreated, and the capillary tool is lowered to press the lower ball end of the wire against the first bonding portion for bonding thereto.
(4) Then, after raising and bringing the capillary tool to a position directly above a second bonding portion (e.g. a lead or a printed conductor pattern) of the workpiece, the capillary tool is lowered again to press a suitable intermediate portion of the wire against the second bonding portion for bonding thereto.
(5) Finally, the capillary tool is raised again to tear-cut the wire to leave a segment thereof extending between the first and second bonding portions.
The metal wire, which is typically made of gold or aluminum, is generally provided with an insulating resin coating of e.g. polyimide. Such a coating mechanically reinforces the metal wire which is extremely thin and protects the wire against oxidation.
The insulating resin coating does not pose any problem in performing the first bonding wherein the wire lower end is melted into a ball before attachment to the first bonding portion. This is because a portion of the resin coating is melted away at the wire ball end due to the high temperature caused by the spark torch for melting the wire.
However, no melting of the wire nor the resin coating takes place at the time of performing the second bonding wherein the intermediate portion of the wire is attaches to the second bonding portion. Thus, the presence of the resin coating hinders direct metal to metal contact between the wire and the second bonding portion, so that the resulting bonding strength at the second bonding portion may be weak or the electrical connection thereat may be improper.